Red light cameras: How accurate are they?

It's a horrifying moment for California drivers: The flash of a red-light camera as they sail through a "I-swear-it-was-yellow" traffic signal.

That horror turns to dread a week or two later when an ominous envelope from the local traffic enforcement agency arrives in the mail, and very quickly the dread devolves to fist-shaking fury when the envelope is opened to reveal a $490 ticket.

I recently joined the ranks of angry alleged red light violators. I remember the incident: It was about 9:30 p.m. and I was driving along Fifth Street in San Francisco and approaching the intersection with Mission Street. The light was yellow. I kept driving. A bright flash from the red-light camera startled me, and I briefly broke out into a cold sweat. Nah, I thought. It must have been the car behind me. The light was yellow.

The San Francisco Police Department disagreed, and the notice sent in the mail says I ran the red light by a half second. Which led me to wonder: How could this technology be so precise, on a dark and foggy San Francisco night? And if all technology fails at some point -- apps crash, websites go dark and text messages are never received -- can I really believe a machine that claims to have recorded my car within a half second of a red light?

The answer, from industry experts and state traffic safety officials, is mostly, "Yes."

"In general, the safety community considers these machines to be reliable," said Offer Grembek, associate director for research at the UC Berkeley Safe Transportation Research and Education Center.

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And Wei-Bin Zhang of the UC Berkeley Institute of Transportation Studies, said, "I think that the technology itself is very solid."

The California Supreme Court agrees. On Thursday, the court issued a ruling in San Francisco that says that images and data automatically recorded by the cameras have a "presumption of authenticity" similar to other types of photos and videos.

Ironically, the biggest concern with red-light camera systems is that they are so precise. They measure a driver's speed and exact location within a fraction of a second -- but do not leave any wiggle room for the errors of traffic signals such as inconsistent yellow light times and the many variables that every driver brings to an intersection, such as car size, speed, weather and time of day.

"They're almost too accurate," said Jay Beeber, executive director of Safer Streets L.A., a grassroots organization that led successful efforts to end that city's red-light camera program. "You can't have a system that measures things to less than a blink of an eye" but not set up the traffic signals the same way.

Red-light cameras have a few parts: sensors that detect the car's location, cameras to record photos and video, wires that connect the camera to the traffic signal and activate the camera when the signal turns red, and software that collects, records and transmits the data back to the company's processing center where employees review the photos and data before sending it to local police to issue tickets. Redflex, American Traffic Solutions and Xerox have the bulk of red-light camera contracts in California, and Redflex has the lion's share with cameras in 39 cities.

Companies and traffic experts agree it's nearly impossible for a red-light camera to snap a photo of a driver before the light has turned red. The camera is hooked up to the traffic signal controller and will only turn on after the signal is red; Redflex cameras allow for a one-tenth of a second delay after the red light begins.

"One of the urban myths drivers say is that 'I was in the middle of the intersection and the light turned and I got a ticket,' " said Charles Territo, spokesman for ATS. "It doesn't happen."

Hmmmmm. Sounds familiar.

The real problem, some argue, is with the yellow light. Some critics say the it's too short -- the state requires that yellow lights are a minimum of 3.6 seconds in a 35-mile-per-hour zone. But a light may fluctuate a tenth of a second, said Zenon Porche, a sheriff's deputy for Los Angeles County. A yellow light set for 3.6 seconds may actually be on for 3.5 or 3.7 seconds, he said. If shortened by a tenth of a second, the red-light camera would kick on at 3.5 seconds, and the driver would be slapped with a ticket.

"If you're putting in a camera that measures to the tenth of a second, you had better make sure your yellow light is exact," Beeber said.

Porche says he doesn't issue tickets to red light violators when their yellow light is cut short; but that's his personal policy, not the department's. Red-light cameras don't adjust for malfunctioning yellow lights.

"We don't set yellow-light timing and we don't write traffic laws," said Tony Parrino, director of engineering and system support at Redflex.

What difference can a tenth of a second make? A lot, it turns out. West Hollywood increased yellow-light times by three tenths of a second at some intersections and saw a 48 percent to 70 percent drop in red light violations, according to Safer Streets L.A.

Redflex and ATS said their cameras have internal checks that send an alert if any part of the system isn't working, and occasionally things do need to be fixed -- the cameras stop working or the sensors break down.

"Anything can fail," Parrino said. "Every piece of our technology is prone to some type of inability to operate in a specific circumstance."

Many companies are starting to use radar sensors, which are placed on a pole and detect when a car is approaching an intersection, and again when it drives into the intersection. But companies such as Redflex still use loop sensors with some cameras, an aging technology that requires laying wire under the pavement. Loop sensors can be problematic if they're not properly maintained -- they might not sense the car or think a car is there when it isn't, and provide false data, some experts say

"A lot of loop sensors do not work," Zhang said. "There could be some level of errors that occur."

In climates with very cold weather, which can cause pavement to crack, and in areas with a lot of tectonic shifts that move the Earth -- such as the Bay Area -- loop sensors are more likely to stop working.

But Zhang added that "because of revenue involved in this business and because of the protestors I would think these companies would maintain them."

What makes a red-light camera run?
1. Sensors -- These include loop sensors buried under the pavement several feet before and after the stop line at an intersection; wireless sensors, which are the size of a hockey puck and also placed under the pavement; or radar machines mounted on a pole next to the camera. They detect a vehicle when it approaches and passes through the intersection, and use a basic formula of time and distance to calculate the speed of the car.
2. Cameras -- Intersections have multiple still and video cameras with flashes. Digital photos include the car approaching and in the middle of the intersection, the vehicle license plate, the back of the car, the traffic light, and, in California, also of the driver. A video is also recorded on digital or analog cameras.
3. Signal connection -- Cameras are connected to the city's traffic signal control cabinet with electrical wires. Similar to flipping a light switch, when the signal light turns red, the red light camera is turned "on" and can start taking pictures.
4. Processing -- Computers collect all the data, including the pictures, location, time, date, and speed of the car, and encrypt it to send it to the company's processing center (both Redflex and ATS are in Arizona). There, employees sort through data and decide which ones are red light violations. They are then sent to the local law enforcement, who also review the violations and decide which ones to ticket. Then the $490 fines are mailed out.